Nodes to the grindstone

Abstract

Materials become superconductors when charge carriers pair up; this pairing is stabilized by an energy gap, the energy cost for creating single-particle excitations. In any new class of superconductors, the momentumspace structure of this energy gap (or superconducting order parameter) is generally considered to be one of the most important clues to the nature of the pairing mechanism. Determining the order parameter is rarely straightforward, and historically, e.g., in the hightemperature cuprate superconductors, many different types of experiments had to be analyzed and compared before a consensus was achieved. It is therefore not surprising that, nearly two years after the discovery of the high-temperature iron-pnictide superconductors, the symmetry and form of the order parameter in these systems are still controversial. However, the degree of apparent disagreement among different experiments on similar samples [1] has raised the question: can the superconducting state of these materials be extraordinarily sensitive to either disorder or other aspects of electronic structure that “tune” the pairing interaction? In a recent article published in Physical Review B[2], B. Muschler and collaborators from the Walther Meissner Institute in Germany, along with Tom Devereaux, Ian Fisher, and collaborators from Stanford in the US provide important clues using Raman spectroscopy.